We propose to study the functional requirements for the expression of several genes in Drosophila that reside within or near centromeric heterochromatin. These loci were chosen for study since previous genetic evidence demonstrated that they act in a manner opposite that typical of a euchromatic gene. Chromosomal rearrangements that remove these genes from their normal proximal position on a chromosome and juxtapose them to distal euchromatic regions lead to partial suppression or variegated expression of the gene. In the proposed research, the light gene and the cubitus interruptus gene will be characterized 1) at the genetic level by isolation of mutations that alter normal expression, 2) at the molecular level by isolation and structural characterization of the genes and surrounding sequences and 3) at the functional level by testing in vivo the requirements for proper expression by transformation. Comparisons of the features of these genes to those typical of euchromatic genes may provide information on the organization of heterochromatin, on the mechanism of position effect and its significance as an indicator of the dependence of gene expression on higher levels of chromosome organization.